1. Field of the Invention
The present invention relates to telecommunications network control. More specifically, the present invention relates to a method, and an accompanying system, for controlling admissions of data packets across a packet network that delivers synchronous services.
2. Background Information
Pseudo wire emulation (PWE) is a mechanism that emulates the essential attributes of a telecommunications service over a packet switched network (PSN). PWE is intended to provide only the minimum necessary functionality to emulate a wire with the required degree of faithfulness for the given service definition. The telecommunication service offered to the customer is delivered by a particular attachment circuit (AC). The service provider edge (PE) equipment translates this AC into a packet based service that can be tunneled across a PSN using the PWE protocol. The tunnel between two PE end-points is referred to as a pseudo-wire.
One of the essential attributes of the telecommunications services described above is the timing synchronization. An exemplary AC is a T1, which has a bit rate of 1.544 Mbps with a clock accuracy of +/−50 parts per million. In a cellular network case, the requirement is 1.544 Mbps with a clock accuracy of +/−50 parts per billion (the increased accuracy is necessary because it determines the accuracy of the radio frequency signal emitted by the cellular transmitter). The telecommunications network is synchronous because each network node learns the timing, i.e., synchronizes, to the preceding node in the hierarchy. The PSN does not have this characteristic. The bit rate of each link is independent. A critical feature of PWE is its ability to reconstruct the timing synchronization of the telecommunications service at the egress from the PSN. If the timing is incorrect, the reconstructed T1 may experience underflow (the data play out too quickly and the T1 runs out of data) or overflow (the data plays out too slowly and the T1 runs out of places to buffer the data and has to discard some of it). Other essential attributes of the telecommunications circuits include low delay and low error rates. Because a low delay is required, the PWE protocol does not use acknowledgements and retries to guarantee that the data successfully transited the PSN. The issue being that, if a retry is required, the replacement data will arrive too late to be forwarded on the reconstructed telecommunications service in a timely manner. Low error rates are required by the telecommunications services because errors show up as audible effects, e.g., crackles, pops, hiss and silence gaps. In the case of PWE, data lost transiting the PSN is replaced by silence or by replay of the data of the previous packet, each of which is an audible impairment of the service.
If the PSN is overloaded (a particularly acute problem when the PSN is composed of wireless links), lost packets will occur frequently. This will degrade the service potentially to the point that it is unusable. Furthermore, timing synchronization is dependent on analysis of the statistical distribution of the arriving packets. If many packets are lost, this analysis will be impossible or incorrect, resulting in timing errors that further degrade the service.